(a) Technical Field
The present disclosure relates, generally, to a motor drive system for a hybrid vehicle and a method for controlling the same. More particularly, it relates to a motor drive system for a hybrid vehicle and a method for controlling the same in the event of a failure in a voltage converter, in which high voltage stored in a DC-link capacitor is suitably discharged to a 12V electrical load through a DC-DC converter, of which the output voltage is increased.
(b) Background Art
Hybrid vehicles are the vehicles of the future that employ an electric motor as an auxiliary power source as well as a gasoline engine to provide a reduction in exhaust gas and an improvement in fuel efficiency.
When the engine operates in an inefficient state, the electric motor is driven by the power of a battery to increase the efficiency of a hybrid system (load leveling). Moreover, the battery is charged by regenerative braking during deceleration, in which the kinetic energy, which would be dissipated as frictional heat in a brake system, is converted into electrical energy by the power generation of the motor, thereby suitably improving the fuel efficiency.
Hybrid vehicles are divided into soft type hybrid vehicles and hard type hybrid vehicles based on whether or not the motor is connected and driven in a power transmission system.
An exemplary motor drive system for an existing hard type hybrid vehicle is shown in FIG. 6. As shown in FIG. 6, the motor drive system preferably includes first and second motors M1 and M2 for driving the vehicle, first and second inverters 1 and 2 for driving the first and second motors M1 and M2, respectively, a DC battery B for outputting a DC voltage, a voltage converter 3 for boosting the DC voltage from the DC battery B and suitably supplying the resulting voltage to the first and second inverters 1 and 2 or for lowering the DC voltage from the first and second inverters 1 and 2 and suitably supplying the resulting voltage to the DC battery B, relay(s) SR1 and SR2 connected between the DC battery B and the voltage converter 3, and a DC-DC converter 4 as an electrical load or power supply device suitably connected between the relay(s) SR1 and SR2 and the voltage converter 3.
The DC-DC converter 4 is commonly called a power converter in which the energy flow is unidirectional or bidirectional. Reference numerals 5, 6 and 7 denote a 12V auxiliary battery, a 12V electrical load, and a DC-link capacitor, respectively.
In the motor drive system for a conventional hybrid vehicle, in the event of a failure in the voltage converter 3, it can take several minutes to hours for the high voltage stored in the DC-link capacitor 7 mounted between the first and second inverters 1 and 2 to be naturally discharged due to the absence of a discharge path, and as a result a repairman may get an electric shock during repair, and a driver or rescuer may get an electric shock in the event of a vehicle accident.
Meanwhile, the voltage supplied from the DC battery B to a DC-DC-DC converter, i.e., the DC-DC converter 4, which is represented as an electrical load, is DC power, and the voltage of the DC battery B is changed according to the state of charge (SOC) of the battery and the amount of energy charged into and discharged from the battery. Accordingly, the DC-DC converter should preferably be configured to cope with the change in battery voltage.
For example, when the voltage of the DC battery B is changed from 200 V to 400 V, while the input current of the DC-DC converter 4 for producing a power of 2 kW is about 5 A at the battery voltage of 400 V, it should be about 10 A at the battery voltage of 200 V. Accordingly, the DC-DC converter is designed based on the current required at the minimum battery voltage, which results in an increase in capacity.
As a result, it is necessary to use a high capacity battery to suitably reduce the change in battery voltage or to minimize the change in SOC of the battery by reducing the amount of energy charged into and discharged from the battery. However, using these two methods may result in an increase in the cost of materials and deterioration in fuel efficiency.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.